1. Field of the Invention
The present invention relates to an anode for use in a lithium ion secondary battery, a method for producing the anode, and a lithium ion secondary battery using the anode.
2. Description of the Related Art
As portable appliances become smaller and have higher performance, secondary batteries for use therein are required to have a higher and higher energy density. Among them, lithium ion secondary batteries are being used as power supplies for the portable appliances, because they show a higher voltage and higher charge/discharge capacity (higher energy density) than nickel-cadmium secondary batteries and nickel-hydrogen secondary batteries.
Such lithium ion secondary batteries are each mainly composed of an anode (negative electrode); a cathode (positive electrode); a separator that insulates between these electrodes; an electrolytic solution that helps the charge transfer between the electrodes; and a battery housing that houses these components. The anode for use in a lithium ion secondary battery is composed of a copper foil or copper alloy foil as a current collector; and an anode active material coated on the current collector. Graphite carbonaceous materials are generally used as the anode active material. However, the discharge capacity of graphite carbonaceous materials reaches the theoretical upper limit (372 mAh/g), and demands have been made to develop anode active materials showing a higher discharge capacity and a higher charge capacity.
One approach to meeting these requirements involves the use of metals that can be alloyed with lithium, such as Si, Ge, Ag, In, Sn, and Pb, as anode active materials showing higher charge/discharge capacity. Typically, Japanese Unexamined Patent Application Publication (JP-A) No. 2002-110151 proposes an anode composed of a current collector and tin (Sn) deposited on the current collector through vapor deposition. This anode shows theoretical charge/discharge capacity of 993 mAh/g which is about 2.5 times that of the graphite carbonaceous material. The anode using Sn, however, suffers from a significantly decreased charge/discharge capacity after repetitive charge and discharge operations, because, during charge/discharge cycles of lithium ion (alloying of Sn with lithium and discharge of lithium), the anode repetitively expands and shrinks in its volume, and thereby the deposited tin film delaminates from the current collector to increase the electrical resistance, or the deposited Sn film itself cracks to increase the contact resistance between Sn film fractions.
To mitigate the volumetric change of the anode active material to thereby solve the above problem, for example, JP-A No. 2004-079463 and JP-A No. 2006-269361 propose anodes each composed of a current collector and an anode active material which is arranged thereon and is composed of an alloy of Sn and a metal that is not alloyed with lithium. JP-A No. 2006-269362 proposes an anode composed of an anode active material; a current collector; and an intermediate layer (diffusion-barrier layer) which is arranged between the anode active material and the current collector and which mitigates the volumetric change of the anode.
However, these known techniques have the following disadvantages. The anodes disclosed in JP-A No. 2004-079463, JP-A No. 2006-269361, and JP-A No. 2006-269362, if used in lithium ion secondary batteries, help to improve the cycle properties. Namely, the anode active materials do not undergo deterioration (delamination or falling off) and can maintain their charge/discharge capacity even after repetitive charge/discharge cycles. All these anodes, however, suffer from a decreased charge/discharge capacity after repetitive charge/discharge cycles, because Sn and the metal that is not alloyed with lithium undergo phase splitting in the anode active material during repetitive charge/discharge cycles, and this ultimately causes cubic expansion and shrinkage of Sn.